Announcement

Collapse
No announcement yet.

Conservation of energy vs Peukert's law

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Conservation of energy vs Peukert's law

    Ok, so we hear a lot about how Peukert's law "steals" from us when we misbehave by applying loads anywhere above what our batteries can deliver at C/20. Then some say Peukert rewards us with an extra 33% of so if we are content with discharging at as little as C/100.

    But what about conservation of energy?

    If you have put 120 AH of your PV's hard-earned power into a 100AH battery to account for losses to ensure a 100 SOC then what happens to the rest if you happily disharge it to zero in only 18mins at 1C? You have only removed about 30AH from the battery in energy terms. Did the rest get converted into heat or some other form of energy? What about when you carefully remove 130AH from the battery at C/100? Was that extra 30AH a free gift from the energy Gods as a reward for being frugal???

    I somehow doubt it. Energy cannot be created or destroyed. My logic tells me that discharging a battery at 1C over 18mins does not yield a 0% real state of charge. It has simply produced an apparent 0% SOC at the plates, which given enough time to rest would yield a 60% SOC once the electrolyte has had time to diffuse. Also a slow discharge at C/100 for 130 hours would surely kill a battery for good as that implies absolutely all of the chemical energy has been removed from the battery. That extra 30AH is what I would call "emergency reserve power" that should never be used under any circumstances if you value your batteries life...

  • #2
    The real capacity is closer to the C/100 rate... depends upon the rate of self discharge.

    When you draw at the C/20 rate, Peukert's law is in effect and you have a lower capacity. The C/20 rate is arbitrary and already includes Peukert effect.

    When you draw at any rate, there are losses (I2R) due to resistance in the battery. The losses go as the square of the current. When you draw down the battery at 1C for 18 minutes, there may be a bit of recovery as diffusion catches up to the current draw, but most of the "missing" energy is lost to heating.

    --mapmaker
    ob 3524, FM60, ePanel, 4 L16, 4 x 235 watt panels

    Comment


    • #3
      Originally posted by mapmaker View Post
      The real capacity is closer to the C/100 rate... depends upon the rate of self discharge.

      When you draw at the C/20 rate, Peukert's law is in effect and you have a lower capacity. The C/20 rate is arbitrary and already includes Peukert effect.

      When you draw at any rate, there are losses (I2R) due to resistance in the battery. The losses go as the square of the current. When you draw down the battery at 1C for 18 minutes, there may be a bit of recovery as diffusion catches up to the current draw, but most of the "missing" energy is lost to heating.

      --mapmaker
      Ah ok so resistence is the thief and heat is the waste product!

      Ok lets see how this could be explained away:

      Assuming a fully charged 12V 100AH battery contains 4.32 MJ of energy and we are only getting 18/60 of that in electrical form at 1C discharge rate, then 3.024 MJ must be converted into heat assuming heat is the only waste product.

      Now to keep this incredibly simple let's assume the entire mass of the battery has the specific heat capacity of water which is 1MJ=240K rise in temperature per kilogram (I know metric isn't very popular in the US but I'm sure all the bright sparks on here can convert easily enough). So 3MJ can raise the temperature of a 26kg battery by:

      (3*240)/26 = 27.69K

      So therefore a 26kg battery discharged at 1C for 18mins would be approximately 27.69 Degrees hotter than it was before being discharged is that correct? 18mins isn't a lot of time for something the size of a 100AH battery to dissapate that heat so I'm going to assume most of that heat will be absorbed by the battery itself. So assuming it started off at 25 degrees it would then be over 50 degrees at the end of this abusive excercise!
      Last edited by Dave3011; 07-02-2014, 08:42 AM. Reason: typo

      Comment


      • #4
        It is burned off as heat during the conversion of electrical to chemical energy. Any conversion of energy has losses.
        MSEE, PE

        Comment


        • #5
          Originally posted by Dave3011 View Post
          A
          So therefore a 26kg battery discharged at 1C for 18mins would be approximately 27.69 Degrees hotter than it was before being discharged is that correct? 18mins isn't a lot of time for something the size of a 100AH battery to dissapate that heat so I'm going to assume most of that heat will be absorbed by the battery itself. So assuming it started off at 25 degrees it would then be over 50 degrees at the end of this abusive excercise!
          Perhaps a bit simplified, but correct. The energy lost to heat will end up primarily in the battery, via both chemical reaction losses and I2R losses in plates and electrolyte. I would not be surprised if the specific heat of lead was also greater than the specific heat of water (by weight).
          SunnyBoy 3000 US, 18 BP Solar 175B panels.

          Comment


          • #6
            Originally posted by inetdog View Post
            I would not be surprised if the specific heat of lead was also greater than the specific heat of water (by weight).
            It's not... not even close. By volume, it is a bit higher than water. --mapmaker
            ob 3524, FM60, ePanel, 4 L16, 4 x 235 watt panels

            Comment


            • #7
              It's an interesting question and Ralph Hiesey at Bogart Engineering, who probably has more years testing batteries than anybody here, tells why he don't use Peukert Number or Factor in his battery monitors:
              http://www.bogartengineering.com/sit...tsComments.pdf

              What Ralph says is true in my experience. We got three different battery monitors on our system here - two that use Peukert compensation, and the TriMetric that doesn't. Time after time the TriMetric is the most accurate one on heavy discharges. I've seen many times when the two monitors that use Peukert Factor say the battery is 25% SOC or lower and the TriMetric says 40% SOC or so. And indeed if I check SG's the result agrees with the TriMetric, and on full recharge the TriMetric is the one that reaches 100% SOC at end amps while the Peukert compensated monitors are only showing 85% SOC or so, and have to "sync" when the battery reaches full charge.
              off-grid in Northern Wisconsin for 14 years

              Comment


              • #8
                On the other hand, when recharging you do have a Coulombmetric efficiency factor which includes the problem that some of the current will end up electrolyzing water instead of running the reverse reaction on the active material and the electrolytes.
                This factor will also be somewhat dependent on the charge rate, but will vary even more between voltages below the gassing voltage versus above the gassing voltage (different stages of the charging process and different IR drops across the internal resistance of the battery.)
                SunnyBoy 3000 US, 18 BP Solar 175B panels.

                Comment


                • #9
                  inetdog - I have that set at 91% in all the meters. I ran like 10 cycles back when we got the TriMetric and in the history it shows what actual losses are on the last five cycles. After you run enough cycles, you can set it to what the actual is, based on how you cycle your batteries. If you cycle shallow the efficiency will be much lower (around 85%). If you cycle very deep to like 20% SOC the efficiency will be very high (around 95%). We typically cycle at 40% SOC over 7-10 days on a cycle, so it worked out to 91%.
                  off-grid in Northern Wisconsin for 14 years

                  Comment


                  • #10
                    Originally posted by ChrisOlson View Post
                    It's an interesting question and Ralph Hiesey at Bogart Engineering, who probably has more years testing batteries than anybody here, tells why he don't use Peukert Number or Factor in his battery monitors:
                    http://www.bogartengineering.com/sit...tsComments.pdf

                    What Ralph says is true in my experience. We got three different battery monitors on our system here - two that use Peukert compensation, and the TriMetric that doesn't. Time after time the TriMetric is the most accurate one on heavy discharges. I've seen many times when the two monitors that use Peukert Factor say the battery is 25% SOC or lower and the TriMetric says 40% SOC or so. And indeed if I check SG's the result agrees with the TriMetric, and on full recharge the TriMetric is the one that reaches 100% SOC at end amps while the Peukert compensated monitors are only showing 85% SOC or so, and have to "sync" when the battery reaches full charge.
                    I am a complete novice when it comes to battery technology so I have to ask the question, do you feel the type of battery or chemistry might be a reason the monitors using the Peukert compensation is not dead on accurate?

                    From experience in other technologies, formulas are accurate when they are based on very specific conditions. Maybe your batteries fall outside those conditions.

                    Comment


                    • #11
                      As far as I know, Ralph's white paper is dealing with FLA, and that's what we got. I'm sure other types would be different. I've seen that when we discharge at a high rate the Peukert Factor does come into play as you don't get as many amp-hours out as the battery is rated at at the 20 hr rate until the voltage drops to 1.75 VPC. BUT - if you reduce the load on the battery and let it recover for a bit, discharging at a much slower rate you can get the rest of the amp-hours out of it. It is not dead.
                      off-grid in Northern Wisconsin for 14 years

                      Comment


                      • #12
                        Originally posted by ChrisOlson View Post
                        As far as I know, Ralph's white paper is dealing with FLA, and that's what we got. I'm sure other types would be different. I've seen that when we discharge at a high rate the Peukert Factor does come into play as you don't get as many amp-hours out as the battery is rated at at the 20 hr rate until the voltage drops to 1.75 VPC. BUT - if you reduce the load on the battery and let it recover for a bit, discharging at a much slower rate you can get the rest of the amp-hours out of it. It is not dead.
                        I understand. I read the paper after I made posted my comments.

                        I also noticed in the last paragraph of the paper he stated "Finally, the other issue about using Perkert's relation, although theoretically interesting, is that it's rather difficult to get an equation that reliably reproduces performance of an actual battery. And unless it is used in applications where the loads vary a lot, it probably wouldn't give a very useful or accurate results...".

                        So while the formula can be accurate, in real life it may not be 100% especially if your discharge decreases allowing the battery to somewhat recover.

                        Comment


                        • #13
                          Yes, Ralph is a brilliant engineer and he put a lot testing into it for real-life off-grid batteries before deciding it wasn't worth it to use Peukert compensation in his monitors. I've talked to him on the phone before and he's an interesting fellow. I think the Peukert number works at constant discharge rates. But few real-life off-grid systems are designed for high constant discharge rates. Most are designed for discharge rates at right around the 100 hour rate, and the high discharges are very intermittent.
                          off-grid in Northern Wisconsin for 14 years

                          Comment

                          Working...
                          X